I am relatively new to this and had a quick question about water amounts. First, I have told myself I will brew 10 basic batches before I start deviating to much, as I want to get the process down before I start doing more advanced brewing. I have bought 4 basic kits that contain LME, specialty grains, hops, and the sugars. The directions state to basically use 2.5 gallons for wort and then to dump the 2.5 gallons of wort into 2.5 gallons of water already in my fermenter (after I cool the wort). Ok, simple enough and I have done so in all my batches so far....but my question is this..

I have a 40L brew pot, doesn't it make more sense to do the entire 5 gals at once? Do I have to boil, steep, and so on with only 2.5 gals? With such a big brew pot it doesn't take long to get a boil.

If your kettle is large enough, a full volume boil (6-ish gals reduces to 5-ish) will make a better beer. But you need a way to cool the larger volume, since you're not diluting, adding ice, etc. Full volume boil will give you better hop utilization, among other benefits.

Yes, they assume that most "noobs" don't have pot big enough. You will have to adjust the hops, the (hops)utilization changes with the addition of the more water in the boil. You can down load a trial version (21 days free) of one of the brewing software programs and figure out the proper hops addition adjustments. I would also suggest a wort chiller if you decide to go with full boils it makes a big differance.

40L is a big pot (about 10 gal.). I assume you will be using a turkey fryer outside as your stove would have a hard time boiling 5+ gallons of water. I steep my grains inside on the stove while my brew pot heats up outside. You don't want to use too much water when steeping your grains (something about extracting tannins). 1.5 -2 gallons for steeping should be good. 4 -4 1/2 for the boil should get you close to the 5 gal. mark when all is said and done. You will probably lose a gallon or more during the boil. Just top it off if needed.
You can get what you need for the wort chiller at the hardware store for @ $25 or so. The cold break is important so I highly recommed it.

@bad67z - I can do the math on the hops and I am in the process of making what I have called a "triple rib cage wort chiller", basically the rib cage design with three sets of coils that essentially form a large triangle in my 40L brewpot. I cannot wait to get my wort chiller working for my next batch.

@Jamison - Actually, while I have a turkey fryer, my propane stove in the house has no problems boiling 5gals in my 40L kettle. I will adjust for evaporation to my 5gals of tasty beer goodness.

Thank you all for answering my questions. This has not been the first nor do I expect this to be my last.